Image processing apparatus and an image processing method for performing a correction process of a tone value of a pixel for smoothing a jaggy

ABSTRACT

With this invention, color shifting correction is performed first based on shifting amount information indicating a shifting amount with respect to the scanning direction on an image carrier of each image forming unit, and halftone processing is then performed, thus suppressing generation of moiré due to the color shifting correction, and forming a high-quality image. To this end, an image forming engine has color shifting amount storage units C, M, Y, and K (black) which store actual shifting amounts with respect to ideal scan directions on image carriers C, M, Y, and K in image forming units C, M, Y, and K. Color shifting correction amount arithmetic units C, M, Y, and K calculate color shifting correction amounts for respective color components on the basis of the stored color shifting amounts. Color shifting correction units C, M, Y, and K perform color shifting correction by converting coordinates upon reading out image data from bitmap memories C, M, Y, and K on the basis of the calculated color shifting correction amounts, and then perform tone correction. Data after tone correction undergo halftone processing by halftone processors. C, M, Y, and K. PWM processors C, M, Y, and K generate PWM signals for scanning, and output them to exposure units C, M, Y, and K of the respective image forming units.

This application is a continuation of U.S. patent application Ser. No.14/049,214, filed Oct. 9, 2013, which is a continuation of U.S. patentSer. No. 12/716,125, filed Mar. 2, 2010, now U.S. Pat. No. 8,570,594,which is a continuation of U.S. patent application Ser. No. 11/274,141,filed Nov. 16, 2005, now U.S. Pat. No. 7,684,079. The contents of eachof the foregoing applications are incorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to a technique for forming a color imageby transferring color formers that form color component images to bedeveloped on a plurality of juxtaposed image carriers onto a conveyedprint medium.

BACKGROUND OF THE INVENTION

Conventionally, as a color image forming apparatus which uses anelectrophotography method, an apparatus which uses a plurality ofdevelopers for one photosensitive body to develop respective colorcomponents is known. This apparatus repeats an“exposure-development-transfer” process a number of times equal to thenumber of color components to overlay and form color images on a singletransfer sheet in these processes, and to fix these color images, thusobtaining a full-color image.

With this method, the image forming process must be repeated threetimes, or four times if black is used, per print image, and thus takes along time to complete image formation.

As a method that can cover this shortcoming, a technique that uses aplurality of photosensitive bodies, overlays visible images obtained forrespective colors in turn on a transfer sheet, and obtains a full-colorprint by a single sheet feed process is known.

With this method, the throughput can be greatly improved. However, colorshifting owing to position shifts of respective colors on the transfersheet occurs due to limitations on the achievable positional precisionof, and diameter shifts (slight shifts in position of the axes) of therespective photosensitive bodies, positional precision shifts of therespective optical systems, and the like, and it becomes difficult toobtain a high-quality full-color image.

As a method of preventing this color shifting, a technique for forming atest toner image on a transfer sheet or a conveyor belt that forms apart of transfer means, detecting that image, and correcting the opticalpaths of respective optical systems and correcting the image write startpositions of respective colors based on the detection result is known(e.g., Japanese Patent Laid-Open No. 64-40956; to be referred to as“reference 1” hereinafter).

Also, a technique for automatically converting the output coordinatepositions of image data for respective colors into those from which anyregistration shifting is corrected, and correcting the positions ofmodulated light beams in an amount smaller than a minimum dot unit ofeach color signal by correction means on the basis of the convertedimage data of the respective colors is known (e.g., Japanese PatentLaid-Open No. 8-85237; to be referred to as “reference 2” hereinafter).

However, with the technique of reference 1, the following problemsremain unsolved.

First, in order to correct the optical paths of the optical systems, acorrection optical system including a light source and f-θ lens, mirrorsin the optical paths, and the like must be mechanically operated toadjust the position of the test toner image. That is, high-precisionmovable members are required, resulting in high cost. Furthermore, sinceit takes a long time to complete the correction, the correction cannotbe frequently made. Also, the optical path lengths often change with thelapse of time due to the temperature rise of the machine. In such case,it is difficult to prevent color shifting by correcting the opticalpaths of the optical systems.

Second, upon correcting the write-start positions of images, theposition shifts of the upper end and upper left portion can becorrected. However, any tilt of an optical system, and any magnificationshifting that may occur due to possible optical path length shifting,cannot be corrected.

In reference 2, as a result of correcting the output coordinatepositions of image data for respective colors for an image that hasundergone halftone processing, dot reproducibility of the halftone imagedeteriorates, color nonuniformity occurs and moiré becomes obvious.

FIG. 1 shows an example, which will be described below. An input image101 has an image having a given density value. Assume that an image 102obtained by applying arbitrary color shifting correction to this inputimage 101 is printed in practice. In this case, since the image densityvalues and toner densities for that image density value have a nonlinearrelationship, although the input image 101 has a constant density value,if the image after color shifting correction is printed, the result isan actual printed image whose density value is not constant. Therefore,when such nonuniform density values appear periodically, moiré becomesobvious, and a high-quality color image cannot be obtained.

Furthermore, in the search for ways to speed up printer engines, it hasbecome common not to stop the photosensitive drum during scanningexposure of a laser beam, but rather to rotate it even during scanningexposure. At this time, if the scanning exposure directions of imageforming units of respective color components are the same, no problem isposed. However, when a given image forming unit scans in a directionopposite to that of another image forming unit, this causes colornonuniformity. Since the scan speed and rotational speed of the drumvary depending on print mode, color shifting cannot be suppressed bymeans of a single countermeasure processing so far.

SUMMARY OF THE INVENTION

The present invention has been made to solve the aforementionedproblems, and has as its object to provide a technique for forming ahigh-quality image by correcting any color shifting first by calculatingthe read coordinate position of image data to be printed on the basis ofshifting-amount information indicating a shifting amount with respect tothe scanning direction on an image carrier of each image forming unit,and then executing halftone processing to print an image, thussuppressing generation of moiré due to color-shifting correction.

In order to achieve the above object, for example, an image formingapparatus of the present invention comprises the following arrangement.That is, there is provided an image forming apparatus in which imageforming units each having an image carrier, an exposure unit forscanning exposure on the image carrier, and a developing unit forvisualizing an electrostatic latent image generated by exposure using acolor former are juxtaposed in a conveying direction of a print medium,characterized by comprising:

image data storage means for storing image data to be formed by eachimage forming unit;

exposure shifting amount storage means for storing shifting amountinformation indicating a shifting amount with respect to a scanningdirection on the image carrier of each image forming unit;

coordinate conversion means for converting coordinates of a read addressof the image data storage means on the basis of the exposure shiftingamount information stored in the exposure shifting amount storage means,and reading out image data according to the converted addressinformation;

correction means for correcting a tone of pixel data read out by thecoordinate conversion means on the basis of the converted addressinformation;

halftone means for applying predetermined halftone processing to thepixel data obtained by the correction means; and

output means for outputting the pixel data obtained by the halftonemeans as an exposure control signal of the exposure unit of thecorresponding image forming unit.

It is an object of the second invention to provide a technique forforming a high-quality image by suppressing generation of jaggednesseven in an edge of a character/line image, in addition to the object ofthe first invention.

In order to achieve the above object, an image forming apparatusaccording to the second invention comprises the following arrangement.That is, there is provided an image forming apparatus in which imageforming units each having an image carrier, an exposure unit forscanning exposure on the image carrier, and a developing unit forvisualizing an electrostatic latent image generated by exposure using acolor former are juxtaposed in a conveying direction of a print medium,characterized by comprising:

image data storage means for storing image data to be formed by eachimage forming unit;

exposure shifting amount storage means for storing shifting amountinformation indicating a shifting amount with respect to a scanningdirection on the image carrier of each image forming unit;

coordinate conversion means for converting coordinates of a read addressof the image data storage means on the basis of the exposure shiftingamount information stored in the exposure shifting amount storage means,and reading out image data according to the converted addressinformation;

buffer means for storing pixel data read out by the coordinateconversion means for a plurality of lines;

determination means for determining, based on pixel data of interest anda surrounding pixel data group stored in the buffer means, if the pixeldata of interest belongs to an image edge;

first processing means for, when the determination means determines thatthe pixel of interest belongs to a non-image edge, applying halftoneprocessing for the non-image edge to the pixel data of interest;

correction means for, when the determination means determines that thepixel of interest belongs to the image edge, correcting a tone of thepixel data of interest stored in the buffer means on the basis ofaddress information used upon conversion by the coordinate conversionmeans;

second processing means for applying processing for an edge differentfrom the first processing means to the pixel data obtained by thecorrection means; and

output means for outputting the pixel data obtained by the first andsecond processing means as an exposure control signal of the exposureunit of the corresponding image forming unit on the basis of thedetermination result of the determination means.

It is an object of the third invention to provide a technique forforming a high-quality image by executing color shifting correctionfirst by calculating the read coordinate position of image data to beprinted using not only an exposure profile indicating a shifting amountwith respect to the scanning direction on an image carrier of each imageforming unit, but also a print profile as configuration information of aprint engine, and then executing halftone processing to print an image,thereby suppressing generation of moiré due to the color shiftingcorrection and also generation of jaggedness even in an edge of acharacter/line image.

In order to achieve the above object, an image forming apparatusaccording to the third invention comprises the following arrangement.That is, there is provided an image forming apparatus in which imageforming units each having an image carrier, an exposure unit forscanning exposure on the image carrier, and a developing unit forvisualizing an electrostatic latent image generated by exposure using acolor former are juxtaposed in a conveying direction of a print medium,characterized by comprising:

image data storage means for storing image data to be formed by eachimage forming unit;

exposure shifting amount storage means for storing shifting amountinformation indicating a shifting amount with respect to a scanningdirection on the image carrier of each image forming unit;

configuration information storage means for storing informationassociated with a configuration of each image forming unit;

coordinate conversion means for converting coordinates of a read addressof the image data storage means on the basis of the exposure shiftingamount information stored in the exposure shifting amount storage meansand the configuration information stored in the configurationinformation storage means, and reading out image data according to theconverted address information;

determination means for determining, based on pixel data of interest anda surrounding pixel data group obtained by the coordinate conversionmeans, if the pixel data of interest belongs to an image edge;

first processing means for, when the determination means determines thatthe pixel of interest belongs to a non-image edge, applyingpredetermined halftone processing;

correction means for, when the determination means determines that thepixel of interest belongs to the image edge, correcting a tone of thepixel data of interest on the basis of the converted addressinformation;

second processing means for applying processing for an edge differentfrom the first processing means to the pixel data of interest aftercorrection by the correction means; and

output means for outputting one of the pixel data obtained by the firstand second processing means as an exposure control signal of theexposure unit of the corresponding image forming unit on the basis ofthe determination result of the determination means.

Other features and advantages of the present invention will be apparentfrom the following description taken in conjunction with theaccompanying drawings, in which like reference characters designate thesame or similar parts throughout the figures thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention.

FIG. 1 is a view showing density nonuniformity in the prior art;

FIG. 2 is a sectional view showing the structure of an image formingapparatus according to an embodiment of the present invention;

FIG. 3 is a graph for explaining shifting of a main scan line scanned ona photosensitive drum in the embodiment of FIG. 2;

FIG. 4 is a block diagram showing the arrangement of a controller andengine in the image forming apparatus of the embodiment of FIG. 2;

FIG. 5 is a table showing an example of information stored in a colorshifting amount storage unit;

FIG. 6 is a view for explaining the operation for correcting a shiftingamount of an integer part of a color shifting correction amount in acoordinate conversion unit;

FIGS. 7A to 7F are views showing the operation for performing colorshifting correction less than a pixel unit by a tone correction unit inthe embodiment of FIG. 2;

FIG. 8 is a block diagram showing the arrangement of a color shiftingcorrection unit in the embodiment of FIG. 2;

FIG. 9 shows examples of images in respective processes when colorshifting correction is performed after halftone processing;

FIG. 10 shows examples of images in respective processes when halftoneprocessing is performed after color shifting correction;

FIG. 11 is a block diagram showing the detailed arrangement of acoordinate counter 801 and coordinate conversion unit 802 in FIG. 8;

FIG. 12 is a block diagram showing the arrangement of a controller andengine in an image forming apparatus according to the second embodimentof the present invention;

FIG. 13 is a block diagram showing the arrangement of a color shiftingcorrection unit in the second embodiment;

FIG. 14 is a view for explaining the reason why normal halftoneprocessing is not performed at an edge portion of a character/line imagein the second embodiment;

FIG. 15 is a flowchart showing switching processing based on an imageedge determination result in the second embodiment;

FIG. 16 is a block diagram showing the arrangement of a controller andengine in an image forming apparatus according to the third embodimentof the present invention;

FIG. 17 is a view showing the relationship between an exposure profileand print profile in the third embodiment;

FIGS. 18A to 18C are views for explaining the relationship between thenumber of beams and exposure tilt;

FIGS. 19A to 19C are views for explaining the relationship between theprint speed and exposure tilt;

FIG. 20 is a block diagram showing the arrangement of a coordinatecounter according to the fourth embodiment of the present invention;

FIG. 21 is a flowchart showing the print processing sequence in thefourth embodiment;

FIG. 22 is a flowchart showing the write processing sequence in acorrection table in the fourth embodiment;

FIG. 23 is a block diagram showing the arrangement of a coordinatecounter according to the fifth embodiment of the present invention;

FIG. 24 is a view showing an example of a pattern to be printed inexposure profile update processing according to the sixth embodiment ofthe present invention;

FIG. 25 is a block diagram showing the arrangement of a coordinatecounter according to the seventh embodiment of the present invention;and

FIG. 26 is a flowchart showing the print processing sequence in theseventh embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will now be describedin detail in accordance with the accompanying drawings.

<First Embodiment>

FIG. 2 is a sectional view showing the structure of an image formingapparatus according to this embodiment.

As shown in FIG. 2, the image forming apparatus of this embodiment hasthe structure of a 4-drum color laser beam printer.

This image forming apparatus mounts a transfer sheet cassette 53 in alower portion on the right side of FIG. 2. Print media (print sheets,transparent sheets, or the like) set in the transfer sheet cassette 53are picked up one by one by a paper feed roller 54 and are fed to imageforming units by a pair of conveying rollers 55-a and 55-b. The imageforming units are provided with a transfer conveyor belt 10 forconveying a print medium. The transfer conveyor belt 10 is tightenedflat by a plurality of rollers in the print medium conveying direction(from the right to left in FIG. 2), and a print medium iselectrostatically attracted onto the conveyor belt 10 at its mostupstream portion. Four photosensitive drums 14-C, 14-Y, 14-M, and 14-K,which serve as drum-shaped image carriers, are linearly arranged to facethe conveyor belt surface, thus forming the image forming units (notethat C, Y, M, and K respectively indicate cyan, yellow, magenta, andblack color components).

Since the image forming units for respective color components have thesame structure except for the colors of toners to be stored, the imageforming unit for the color component C will be described below as anexample.

The C image forming unit has a charger 50-C for uniformly charging thesurface of the photosensitive drum 14-C, a developing unit 52-C forstoring C toner, and visualizing (developing) an electrostatic latentimage generated on the photosensitive drum 14-C, and an exposure unit51-C. A predetermined gap is formed between the developing unit 52-C andcharger 50-C. The surface of the photosensitive drum 14-C which isuniformly charged by the charger 50-C is scanned by a laser beam fromthe exposure unit 51-C including a laser scanner via the gap in adirection perpendicular to the plane of the drawing. As a result, thescanned exposure portion has a charged state different from an unexposedportion, thus forming an electrostatic latent image. The developing unit52-C visualizes the electrostatic latent image by transferring toner toit (“toner image formation”, or “development”).

A transfer unit 57-C is arranged over the conveying surface of thetransfer conveyor belt 10. The toner image formed (developed) on thecircumferential surface of the photosensitive drum 14-C is electricallyattracted on the conveyed print medium by a transfer electric fieldformed by the transfer unit 57, and is transferred onto the print mediumsurface.

The aforementioned processing is similarly repeated for other colorcomponents Y, M, and K, so that C, M, Y, and K toners are transferred inturn onto the print medium. After that, a fixing device 58 fixes thecolor toners on the print medium by thermally melting them, and theprint medium is ejected from the apparatus via a pair of exhaust rollers59-a and 59-b.

Note that the toner images of respective color components aretransferred onto the print medium in the above example. However, tonerimages of the respective color components may be transferred onto thetransfer conveyor belt, and they may be transferred again onto a printmedium (secondary transfer). The transfer belt in this case is called anintermediate transfer belt.

FIG. 3 shows an image to explain shifting of a main scan line scanned onthe photosensitive drum 14-C (or may be M, Y, and K) as an imagecarrier. The horizontal direction (x-axis direction) in FIG. 3 indicatesthe scanning direction of a laser beam, and the vertical direction(y-axis direction) indicates the rotation direction of thephotosensitive drum (which agrees with the convey direction of a printmedium).

In FIG. 3, reference numeral 301 denotes an ideal main scan line.Reference numeral 302 denotes an example of an actual main scan linewhich suffers an upward slope and curvature resulting from thepositional precision and diameter shifts of the photosensitive drum 14and the positional precision shifting of the optical system in theexposure unit 51 of each color.

When such slope and curvature of the main scan line exist in the imageforming unit of any color, a color shifting occurs when a plurality oftoner images are simultaneously transferred onto a transfer medium.

In this embodiment, point A, serving as a scan start position of theprint region, is set as a reference point in the main scan direction(X-direction), and shifting amounts between the ideal main scan line 301and actual main scan line 302 in the sub-scan direction are measured ata plurality of points (points B, C, and D). The main scan line isdivided into a plurality of regions (to define region 1 between Pa andPb, region 2 between Pb and Pc, and region 3 between Pc and Pd) atrespective points where the shifting amounts are measured, and theslopes of the main scan line in respective regions are approximated bystraight lines (Lab, Lbc, and Lcd) which connect neighboring points.Therefore, when the difference (m1 in region 1, m2−m1 in region 2, andm3−m2 in region 3) between the shifting amounts of neighboring points isa positive value, it indicates that the main scan line in the region ofinterest has an upward slope; otherwise, it indicates that it has adownward slope. In this embodiment, the number of regions is three forthe sake of convenience, and the present invention is not limited tosuch specific value.

FIG. 4 is a block diagram for explaining the operation of the colorshifting correction processing for correcting a color shifting generatedby the slope and curvature of the scan line in this embodiment.

Referring to FIG. 4, reference numeral 401 denotes a printer enginewhich performs actual print processing on the basis of image bitmapinformation generated by a controller 402. The controller 402 isaccommodated in a board, and is electrically connected to the printerengine 401 when the board is accommodated in the apparatus.

Reference numerals 403C, 403M, 403Y, and 403K denote color shiftingamount storage units, which receive and hold the shifting amountinformation for respective image forming units of respective colors inthe process of the manufacture of the apparatus. For example, each colorshifting amount storage unit can be implemented by a writable,nonvolatile memory such as an EEPROM or the like. In FIG. 4, the colorshifting amount storage units are assured for respective colorcomponents. However, since the information size to be stored issufficiently small, one memory element may store the color shiftingamounts for all the color components.

The color shifting amount storage units 403C, 403M, 403Y, and 403K ofthis embodiment store shifting amounts between the actual main scan line302 and ideal main scan line 301 in the sub-scan direction, which aremeasured at a plurality of points, as described using FIG. 3, asinformation indicating the slope and curvature of the main scan line.

FIG. 5 shows an example of information stored in the color shiftingamount storage unit 403C (the same applies to the units 403M, 403Y, and403K, but information to be stored varies depending on the individualdifference). In FIG. 5, L1 to L3 and m1 to m3 have the same meanings asthose of the same symbols in FIG. 3.

In this embodiment, the color shifting amount storage units 403C, 403M,403Y, and 403K store the shifting amounts between the ideal main scanline and actual main scan line. However, the present invention is notlimited to such specific amounts as long as information can identify theslope and curvature characteristics of the actual main scan line. Asdescribed above, the information stored in each of the color shiftingamount storage units 403C, 403M, 403Y, and 403K is stored in advance asinformation unique to the apparatus by measuring the shifting amount inthe manufacturing process. Alternatively, a detection mechanism fordetecting the shifting amounts may be prepared in the apparatus itself,and shifting amounts which are obtained by forming predeterminedpatterns used to measure shifts for respective image carriers ofrespective colors, and detecting them by the detection mechanism may bestored.

The controller 402 executes print processing by correcting image datafor respective color components to cancel the shifting amounts of themain scan line stored in the color shifting amount storage units 403C,403M, 403Y, and 403K.

The controller 402 of this embodiment will be described below. An imagegeneration unit 404 generates raster image data that allows printprocessing on the basis of print data (PDL data, image data, or thelike) received from an external apparatus (e.g., a computer apparatus;not shown), and outputs RGB data (8 bits/color, 256 tones) forrespective pixels. Since this processing is known to those who areskilled in the art, a detailed description thereof will be omitted.

A color conversion unit 405 converts this RGB data into data (8bits/color) on a CMYK space that can be processed by the printer engine402 (this conversion is implemented by LOG conversion and UCRprocessing), and stores the converted data in subsequent bitmap memories406C, 406M, 406Y, and 406K for respective print color components. Thebitmap memory 406C (the same applies to the memories 406M, 406Y, and406K) temporarily stores raster image data to be printed, and comprisesa page memory for storing image data for one page. Alternatively, a bandmemory that stores data for several lines may be used. In the followingdescription, assume that each memory has a capacity for storing C, M, Y,or K bitmap data for one page for the sake of simplicity.

Color shifting correction amount arithmetic units 407C, 407M, 407Y, and407K calculate color shifting correction amounts in the sub-scandirection on the basis of the information of the color shifting amountsof the main scan line stored in the color shifting amount storage units403C, 403M, 403Y, and 403K in accordance with the coordinate informationin the main scan direction. The color shifting correction amountoperation units 407C, 407M, 407Y, and 407K respectively output theircalculation results to color shifting correction units 408C, 408M, 408Y,and 408K, which set the corresponding correction amounts.

Let x (dots) be coordinate data in the main scan direction, and y (dots)be the color shifting amount in the sub-scan direction. In this case,arithmetic formulas of the respective regions based on FIG. 3 aredescribed by (assume that the print resolution in this embodiment is 600dpi):Region 1: y=x*(m1/L1)Region 2: y=m1*23.622+(x−L1*23.622)*((m2−m1)/(L2−L1))Region 3: y=m2*23.622+(x−L1*23.622)*((m3−m2)/(L3−L2))  (1)where L1, L2, and L3 are the distances (unit: mm) from the scan startposition of the print region to the right ends of regions 1, 2, and 3.Also, m1, m2, and m3 are the shifting amounts between the ideal mainscan line 301 and actual main scan line 302 at the right ends of regions1, 2, and 3.

The color shifting correction units 408C, 408M, 408Y, and 408K adjustthe output timings of the bitmap data stored in the bitmap memories406C, 406M, 406Y, and 406K and exposure amounts for respective pixels onthe basis of the color shifting correction amounts calculated forrespective dots by the color shifting amount arithmetic units 407C,407M, 407Y, and 407K, so as to correct color shifts due to the slopesand distortions of the main scan line given by formulas (1), therebycolor shifts (registration shifts) upon transferring the toner images ofrespective colors onto a transfer medium.

The color shifting correction units 408C, 408M, 408Y, and 408Krespectively have different correction amounts but the samearrangements. Hence, the color shifting correction unit 408C for the Ccomponent will be described below.

FIG. 8 is a block diagram showing the arrangement of the color shiftingcorrection unit 408C of this embodiment.

As shown in FIG. 8, the color shifting correction unit 408C of thisembodiment comprises a coordinate counter 801, coordinate converter 802,line buffer 803, and tone corrector 804.

The coordinate counter 801 outputs information required to generatecoordinates in the main scan and sub-scan directions, where the colorshifting correction processing is to be executed, on the basis offormulas (1) to the coordinate converter 802, and outputs informationindicating the degree of shifting in the sub-scan direction (a valueafter the decimal point, as will be described later) to the tonecorrector 804.

The coordinate converter 802 makes read access to the bitmap memory 406Cusing coordinate position data (X-address) in the main scan directionand coordinate position data (Y-address) in the sub-scan direction fromthe coordinate counter 801. As a result, read-out data (C component datain this case) is output to the line buffer 803.

The line buffer 803 comprises a register 805 and a FIFO buffer 806having a storage area for one line, as shown in FIG. 8, and outputs Ccomponent data of two neighboring pixels in the sub-scan direction tothe tone corrector 804, which applies tone correction to these data.

FIG. 11 shows a practical example of the coordinate counter 801 andcoordinate converter 802 of this embodiment.

As a precondition, the color shifting correction amount arithmetic unit407C calculates, based on the distances L1, L2, and L3 (unit: mm) storedin the color shifting correction amount storage unit 403C, pixelpositions L1 ′, L2 ′, and L3 ′ in the horizontal direction (ideal scandirection) corresponding to L1, L2, and L3. Also, the color shiftingcorrection amount arithmetic unit 407C calculates the slopes of thestraight lines that connect the shifting amounts of respective regions.Note that the slope is the one for each pixel, and is expressed by Δy.

In case of the example of FIG. 5, we have:Region 1: Δy1=m1/L1Region 2: Δy2=(m2−m1)/(L2−L1)Region 3: Δy3=(m3−m2)/(L3−L2)

A register 82 in FIG. 11 stores the pixel positions L1′, L2′, and L3″,and a register 84 stores Δy1, Δy2, and Δy3 (with a positive/negativesign) of the respective regions.

An X-address generator 81 is reset upon generating correction data forone scan of a laser beam, and generates a read address in the horizontaldirection, i.e., X-address, for the bitmap memory 406C by adding pixelclocks clk. As a result, the X-address increments like 0, 1, 2, . . .every time the pixel clock clk is input.

A comparator 83 compares the X-address value from the X-addressgenerator 81 with registers L1′, L2′, and L3′ to see within which ofregions 1, 2, and 3 in FIG. 3 the current X-address falls, and outputsthe result. Since three states can be taken, an output signal sufficesto be 2 bits.

A selector 85 selects and outputs one of the slopes Δy1, Δy2, and Δy3stored in the register 84. That is, when the current X-address fallswithin the range of region 1 (X≦X L1 ′), Δy1 is selected and output.When L1<X≦L2′, Δy2 is selected and output; when L2′<X, Δy1 is selectedand output.

A counter 86 is reset prior to one scan, cumulatively adds the slope Δyoutput from the selector 85 in an internal register 86 a, and holds thatvalue. Since the slope Δy includes a decimal part, this register 86 ahas an appropriate number of bits. The counter 86 outputs the integerpart of the register 86, which is held by itself to a Y-addressgenerator 87, and the decimal part to the tone corrector 804.

The Y-address generator 87 is set with a reference Y-address in thebitmap memory 406C prior to one scan, adds the reference Y-address andthe integer part from the counter 86, and generates the result as a readY address for the bitmap memory 406C.

As a result, X- and Y-addresses of integers in formulas (1) can begenerated, and C component data at the corresponding position can beread out to the line buffer 803.

A more practical example will be described below. Assume that thereference Y-address is “100”. That is, it is a case wherein data isgenerated for the 100th scan. Also, assume that the value stored in theregister 86 a in the counter 86 is “0.1”.

At this time, pixel data which is located at the Y-coordinate position“100.1” in the bitmap memory 406C is to be ideally loaded. However,since the pixel position of the bitmap memory 406C is expressed by aninteger, the Y-coordinate “100.1” does not exist. From another point ofview, the coordinate “100.1” can be considered as being located betweenaddresses “100” and “101”, so that 90% of its pixel value to becalculated (that after tone correction) is influenced by the pixel valueof the address “100”, and the remaining 10% is influenced by that of theaddress “101”. That is, a value after tone correction can be calculatedusing a weighting coefficient depending on a value indicated by thedecimal part. That is, such value can be given by:H _(x,y) =C _(x,y) *β+C _(x,y+1)*α  (2)

Let γ be the decimal part value output from the counter 86. Then, α andβ have relations given by:β=1−γα=γ

The tone corrector 804 in FIG. 8 executes the aforementioned processing.The tone corrector 804 receives the decimal part value γ output from thecounter 86, calculates correction coefficients α and β to be multipliedby multipliers 804 a and 804 b, and make these multipliers 804 a and 804b multiply α and β, and γ, respectively. By adding these products by anadder 804 c, formula (2) above is calculated, thus outputting thetone-corrected data.

Note that the reference Y-address is incremented by “1” for every scan,but the color shifting correction amount for that reference Y-address,i.e., an offset amount remains the same.

Let P and Q be the X- and Y-addresses generated by the coordinateconverter 802, and the offset of that Y-address be 0.1. Then, theregister 805 loads data at coordinates (P, Q) of the bitmap memory 406C.In this case, the pixel position to be referred to in the interpolationprocessing (P, Q+1), and if the register 805 is considered as the pixelposition of interest, data at the coordinates (P, Q+1) is not loadedyet.

In this connection, this embodiment has the following relationship: datato be output from the FIFO buffer 806 is C component data of the pixelof interest (P, Q), and data to be output from the register 805 is (P,Q+1), as shown in FIG. 8. As described above, since the offset amount ofthe Y-address remains the same for every scan, tone interpolation can beattained using the decimal part value from the coordinate counter 801.

The arrangement and operation of the color shifting correction unit 408Cin this embodiment have been explained, and a further detaileddescription thereof will be given with reference to FIG. 6.

In FIG. 6, reference numeral 60 denotes a color shifting curve which isplotted on the basis of information stored in the color shifting amountstorage unit 403C. The slope of region 1 is Δy1, and that of region 2 isΔy2.

Reference numeral 61 denotes the data storage state in the bitmap memory406C; and 62 (FIG. 6), an exposure image upon exposing image data thathas undergone the color shifting correction for respective pixels on theimage carrier. Also, the positive direction of the sub-scan of thebitmap memory 406C agrees with the down direction with respect to theplane of the drawing, as indicated by reference numeral 61.

As shown in FIG. 6, while the X-address is being updated, Δy1 iscumulatively added in turn. However, since no carry to an integer digitoccurs before address Xa, the Y-address keeps indicating the n-th line.

When address Xa is reached, a carry to an integer digit occurs, and theY-address is updated to indicate the (n+1)-th line.

This integer carry occurs when the X-address in FIG. 6 is Xb, Xc, Xd, .. . . Note that a carry occurs at different periods in regions 1 and 2.This is because these regions have different slopes.

FIGS. 7A to 7F show images for explaining color shifting correction lessthan a pixel unit, i.e., the operation contents for correcting ashifting amount of the decimal part of the color shifting correctionslope Δy by the tone corrector 804 in this embodiment. A shifting amountof the decimal part is corrected by adjusting the exposure ratios of twoneighboring dots in the sub-scan direction.

FIG. 7A shows an image of a main scan line having an upward slope. FIG.7B shows a bitmap image of a horizontal line before tone correction, andFIG. 7C shows a correction image used to cancel any color shifting dueto the slope of the main scan line shown in FIG. 7A. In order togenerate the correction image in FIG. 7C, the exposure amounts of twoneighboring dots in the sub-scan direction are adjusted. FIG. 7D is atable showing the relationship of the correction slope Δy of the colorshifting and the correction coefficients used to attain tone correction.k is an integer (truncating the decimal part) of the color shiftingcorrection amount Δy, and represents a correction amount in the sub-scandirection for each pixel. β and α are correction coefficients used toapply correction less than a pixel unit in the sub-scan direction, andtheir relationship is as described by formula (2) above. That is α is adistribution factor for a preceding dot (data output from the register805 in FIG. 8) and β is that for the dot of interest.

FIG. 7E shows a bitmap image after tone correction for adjusting theexposure ratios of two neighboring dots in the sub-scan direction. FIG.7F shows an exposure image of the tone-corrected bitmap image on theimage carrier. In FIG. 7F, the slope of the main scan line is canceled,and a horizontal straight line is formed.

The color shifting correction unit 408C of this embodiment has beenexplained. Since the same applies to the color shifting correction units408M, 408Y, and 408K of other color components M, Y, and K, color shiftsamong print colors can be set to be less than one pixel at a maximum.

The color-shifting and tone corrected data output from the colorshifting correction units 408C, 408M, 408Y, and 408K undergo halftoneprocessing using predetermined halftone patterns in subsequent halftoneprocessors 409C, 409M, 409Y, and 409K, and then undergo pulse widthmodulation processing in PWM processors 410C, 410M, 410Y, and 410K.These data are then output to the printer engine 401, thus performingexposure processing on the image carriers.

As described above, correction amounts for correcting shifting amountsin the sub-scan direction at respective main scan positions arecalculated from an image bitmap, and are re-constructed as a correctedimage bitmap, thus generating an image from which a color shifting dueto the slope and distortion of the main scan line have been eliminated.

Comparison results upon executing processing in the order of halftoneprocessing→color shifting correction with respect to an input image andupon executing processing in the order of color shiftingcorrection→halftone processing with respect to an input image will bedescribed below.

FIG. 9 shows an example upon executing processing in the order ofhalftone processing→color shifting correction with respect to an inputimage. In FIG. 9, reference numeral 900 denotes an input image with aconstant density of 50%. When the input image 900 undergoes halftoneprocessing using a given 4×4 halftone pattern, an image 901 is obtained.This image 901 is the one to be obtained. However, when an imageequivalent to the image 901 is obtained even after color shiftingcorrection is applied to the image 901, the color shifting correctionfree from image deterioration can be implemented. When the image afterthe halftone process undergoes ½ pixel color shifting correction in theup direction (vertical direction), an image denoted by reference numeral902 in FIG. 9 is obtained. As can be seen from FIG. 9, when the imageafter the halftone processing undergoes the color shifting correction,the reproducibility of halftone dots of the halftone image generated bythe halftone processing deteriorates.

By contrast, FIG. 10 shows an example upon executing processing in theorder of color shifting correction→halftone processing with respect toan input image. In FIG. 10, reference numeral 100 denotes an inputimage, which has a constant density (50%) as in the aforementioned image900. An image 101 is obtained when ½ pixel color shifting correction inthe up direction (vertical direction) is applied to this input image100. As a result of the color shifting correction, images with a densityof 25% are formed on the uppermost and lowermost line portions. An image102 in FIG. 10 is obtained as a result of the halftone processingapplied to this image after the color shifting correction. The image 102is substantially the same as the image 901 except for the uppermost andlowermost lines. In the image 102, no halftone dot deterioration of thehalftone image which is observed in the image 902 is observed, and ahigh-quality color image can be obtained.

Note that the halftone processing in this embodiment generates 4×4 (m×nin general) patterns from input image data. Since 4×4, 16 different toneexpressions are possible. Four-bit (16-tone) multivalued data isassigned to one grid of the 4×4 pattern, and undergoes PWM processing,the 4×4 pattern can consequently express 256 tones.

In this embodiment, the arrangement of the color shifting correctionunit 408C (the same applies to other color components) has beenexemplified using FIGS. 8 and 11. In case of the arrangement of FIG. 11,the offset (shifting) amount of the Y-address is obtained bycumulatively adding Δy in turn in the register 86 a in the counter 86.The arithmetic precision of the decimal part of the register 86 a ispreferably as high as possible. In other words, when the number of bitsof the register 86 a is small, a round error gradually occurs duringcumulative addition of Δy, and the register value deviate from the pathsof the slopes Δy1 and Δy2 in FIG. 6.

Therefore, every time the X-address used to load data from the bitmapmemory 406C is updated, the offset amount of the Y-address may becalculated according to formulas (1). Since no round error due tocumulative addition occurs, pixel data at positions indicated by thenormal paths can be read out.

The arrangement shown in FIG. 4 can also be implemented by software(firmware). In this case, processing that allows image data to flowaccording to FIG. 4 can be implemented, and such implementation is easyfor those who are skilled in the art from the description of thisembodiment.

As described above, according to the first embodiment, color shiftingcorrection is done first by calculating the read coordinate position ofimage data to be printed on the basis of shifting amount informationindicating the shifting amount with respect to the scanning direction onthe image carrier of each image forming unit, and halftone processing isthen executed to print an image, thus suppressing generation of moirédue to color shifting correction, and forming a high-quality image.

<Second Embodiment>

The second embodiment will be described below.

FIG. 12 is a block diagram for explaining the operation of the colorshifting correction processing for correcting any color shiftinggenerated due to the slope and curvature of the scan line in the secondembodiment. The difference between FIG. 4 in the first embodiment andFIG. 12 is that the color shifting correction units 408C, 408M, 408Y,and 408K are replaced by units 408C′, 408M′, 408Y′, and 408K′. Also, inaddition to the color shifting correction units 408C, 408M, 408Y, and408K, exception processors 411C, 411M, 411Y, and 411K are added, andselectors 412C, 412M, 412Y, and 412K each for selecting one of outputsof the halftone processors 409C, 409M, 409Y, and 409K and those of theexception processors 411C, 411M, 411Y, and 411K are added.

Other arrangements are the same as those of the first embodiment, anddifferences will be explained below.

The color shifting correction units 408C′, 408M′, 408Y′, and 408K′respectively have different correction amounts but the samearrangements. Hence, the color shifting correction unit 408C′ for the Ccomponent will be described below.

FIG. 13 is a block diagram of the color shifting correction unit 408C′in the second embodiment. The same reference numerals in the arrangementof FIG. 13 denote the same parts as in the arrangement of FIG. 4 of thefirst embodiment.

The color shifting correction unit 408C′ of the second embodimentcomprises a coordinate counter 801, coordinate converter 802, linebuffer unit 1803, edge pattern memory 1805, edge detector 1806, and tonecorrector 804. Of these components, the coordinate counter 801,coordinate converter 802, and tone corrector 804 are the same as thosein FIG. 4.

As in the first embodiment, the coordinate counter 801 outputsinformation required to generate coordinates in the main scan andsub-scan directions, where the color shifting correction processing isto be executed on the basis of formulas (1), to the coordinate converter802, and outputs information indicating the degree of shifting in thesub-scan direction (a value after the decimal point, as will bedescribed later) to the tone corrector 804.

As in the first embodiment, the coordinate converter 802 makes readaccess to the bitmap memory 406C using coordinate position data(X-address) in the main scan direction and coordinate position data(Y-address) in the sub-scan direction from the coordinate counter 801.As a result, read-out data (C component data in this case) is output tothe line buffer unit 1803.

The line buffer unit 1803 includes three line buffers 1803 a, 1803 b,and 1803 c, as shown in FIG. 13, and outputs a 3×3 window 1804 includingpixel data of interest (data obtained by coordinate conversion) to theedge detector 1806.

The edge detector 1806 compares the input 3×3 window data and a patternstored in the edge pattern storage unit 1805, and checks if the pixel ofinterest at the center of the window belongs to an edge portion of acharacter/line image or the like. If it is determined that the pixel ofinterest belongs to an edge portion of a character/line image, the edgedetector 1806 outputs a pixel of interest Pn(x) (the line buffer 1803 bthat stores image data of the n-th line) and pixel data Pn+1(x) at thesame main scan coordinate position of the (n+1)-th line (the line buffer1803 a) to the tone corrector 804, which executes tone correction.

On the other hand, if it is determined that the pixel of interest doesnot belong to an edge of a character/line image, i.e., if it isdetermined that the pixel of interest belongs to a tone image such as aphoto image or the like, the tone correction is skipped, and halftoneprocessing is executed by the halftone processor 409C.

At this time, a signal indicating whether or not the edge detector 1806detects an edge, i.e., if a matching pattern in the edge pattern memory1802 is found is output to the selector 412C. As a result, the selector412C selects one of data from the exception processor 411C and halftoneprocessor 409C, and outputs the selected data.

The processing of the color correction unit 408C′ of the secondembodiment has been described. The same applies to the color shiftingcorrection units 408M′, 408Y′, and 408K′ of other color components.

Note that an object which is to undergo tone correction by the tonecorrector 804 is an edge portion of a character/line image or the likeaccording to the second embodiment.

The exception processors 411C, 411M, 411Y, and 411K of the secondembodiment will be described below.

A case upon executing processing in the order of halftoneprocessing→color shifting correction with respect to an input image anda case upon executing processing in the order of color shiftingcorrection→halftone processing with respect to an input image will beexamined below.

FIG. 9 shows an example upon executing processing in the order ofhalftone processing→color shifting correction with respect to an inputimage. In FIG. 9, reference numeral 900 denotes an input image with aconstant density of 50%. When the input image 900 undergoes halftoneprocessing using a given 4×4 halftone pattern, an image 901 is obtained.This image 901 is the one to be obtained. However, when an imageequivalent to the image 901 is obtained even after color shiftingcorrection is applied to the image 901, the color shifting correctionfree from image deterioration can be implemented. When the image afterthe halftone process undergoes ½ pixel color shifting correction in theup direction (vertical direction), an image denoted by reference numeral902 in FIG. 9 is obtained. As can be seen from FIG. 9, when the imageafter the halftone processing undergoes the color shifting correction,the reproducibility of halftone dots of the halftone image generated bythe halftone processing deteriorates.

By contrast, FIG. 10 shows an example upon executing processing in theorder of color shifting correction→halftone processing with respect toan input image. In FIG. 10, reference numeral 100 denotes an inputimage, which has a constant density (50%) as in the aforementioned image900. An image 101 is obtained when ½ pixel color shifting correction inthe up direction (vertical direction) is applied to this input image100. As a result of the color shifting correction, images with a densityof 25% are formed on the uppermost and lowermost line portions. An image102 in FIG. 10 is obtained as a result of the halftone processingapplied to this image after the color shifting correction. The image 102is substantially the same as the image 901 except for the uppermost andlowermost lines. In the image 102, no halftone dot deterioration of thehalftone image which is observed in the image 902 is observed, and ahigh-quality color image can be obtained.

That is, in case of an image having no edge like the images 900 and 100,image deterioration can be suppressed by applying halftone processing toan image that has undergone color shifting correction.

On the other hand, in case of an image edge portion whose densitychanges abruptly with respect to a surrounding portion like a character,line image, or the like, as shown in FIG. 14, since an edge portion isformed in accordance with a halftone pattern by halftone processing,tone correction is invalidated, and gaps and discontinuities aregenerated at an edge portion of an image generated by exposure suffersgap, like reference symbol 1100 in FIG. 14. As a result, jaggy isgenerated at the image edge portion of a character/line image, or thelike.

In order to prevent this, exception processing is applied to an imageafter color shifting correction for the image edge portion of acharacter/line image, or the like.

The exception processor 411C (the same applies to the processors 411M,411Y, and 411K) executes exception processing different from normalhalftone processing for an image from which an edge is detected by theedge detector 1806.

There are three types of exception processing, as follows.

-   -   No halftone processing is applied (through). In this case, since        no halftone processing is applied to an image from which an edge        is detected by the edge detector 1806, gaps and discontinuities        caused at an edge portion by the halftone processing can be        prevented.    -   Halftone processing is applied using a halftone pattern for an        edge portion. When a normal halftone pattern is used at the edge        portion, as shown in FIG. 14, gaps and discontinuities are        generated depending on the growth direction of the halftone        pattern. Hence, when a halftone pattern having a growth        direction from a normal one is used for the edge portion, gaps        and discontinuities generated using the normal halftone pattern        can be prevented.    -   Processing for compensating for dots after normal halftone        processing or the like is executed. After the normal halftone        processing, dots are compensated for gaps and discontinuous        portions to compensate for the gaps and discontinuities. In this        way, any gaps and discontinuities generated by the normal        halftone processing can be compensated for.

By contrast, the halftone processor 409C (the same applies to theprocessors 409M, 409Y, and 409K) applies normal halftone processing toan image with a non-edge portion.

The flow of a series of processes can be executed, as shown in FIG. 15.

In step S121, coordinate conversion is executed using the coordinateconverter 802 to correct a color shifting equal to or larger than oneline.

In step S122, the converted data obtained by the coordinate converter802 is stored in the line buffer unit 1803.

In step S123, the edge detector 1806 detects an edge portion of acharacter/line image or the like. If an edge is detected, the flowadvances to step S124; otherwise, the flow advances to step S125.

In step S124, the tone corrector 804 applies tone correction to an imagewith an edge portion to execute color shifting correction less than onepixel. Then, exception processing in step S126 is executed. That is,exception processing such as halftone processing using a halftonepattern different from a normal pattern, processing for adding dots todiscontinuous portions and gaps generated by halftone processing, or thelike is executed.

On the other hand, if an image with a non-edge is detected, halftoneprocessing is executed in step S125.

Pulse width modulation is made on the basis of image data obtained fromone of the aforementioned exception processor 411C or halftone processor409C to be converted into a binary laser drive signal, which is thensupplied to an exposure unit to make exposure. The same processing as inthe above processing is similarly applied to other color components M,Y, and K.

As described above, according to the second embodiment, color shiftingcorrection is done first by calculating the read coordinate position ofimage data to be printed on the basis of shifting amount informationindicating the shifting amount with respect to the scanning direction onthe image carrier of each image forming unit. After that, halftoneprocessing is then executed to print an image, thus suppressinggeneration of moiré due to color shifting correction. Furthermore, asfor an edge of a character/line image, generation of jaggedness can besuppressed, and a high-quality image can be formed.

<Third Embodiment>

The third embodiment will be described below.

FIG. 16 is a block diagram for explaining the operation of the colorshifting correction processing for correcting any color shiftinggenerated due to the slope and curvature of the scan line in the thirdembodiment. The difference between FIG. 12 in the second embodiment andFIG. 16 is that the engine 401 comprises exposure profile storage units1403C, 1403M, 1403Y, and 1403K, and a print profile storage unit 1420.Based on this, color shifting correction amount arithmetic units 1407C,1407M, 1407Y, and 1407K are arranged.

The exposure profile storage units 1403C, 1403M, 1403Y, and 1403K storethe same data as in the color shifting amount storage units 403C, 403M,403Y, and 403K in the first and second embodiments. That is, theexposure profile storage units 1403C, 1403M, 1403Y, and 1403K receiveand hold the shifting amount information for respective image formingunits of respective colors in the manufacturing process of theapparatus. For example, each exposure profile storage unit can beimplemented by a writable, nonvolatile memory such as an EEPROM or thelike. In FIG. 16, the exposure profile storage units are assured forrespective color components. However, since the information size to bestored is sufficiently small, one memory element may store the colorshifting amounts for all the color components.

The print profile storage unit 1420 stores configuration informationassociated with print processing in the printer engine 401. The printprofile storage unit 1420 also comprises a writable, nonvolatile memory.

The color shifting correction amount arithmetic unit 1407C (the sameapplies to the units 1407M, 1407Y, and 1407K) calculates a colorshifting correction amount on the basis of data from the exposureprofile storage unit 1403C and print profile storage unit 1420.

Since arrangements other than those described above are the same as thesecond embodiment, the same reference numerals denote such components,and refer to the first and second embodiment for a description thereof.

The exposure profile storage unit 1403C (the same applies to the units1403M, 1403Y, and 1403K, but information to be stored differs dependingon individual differences) stores the same data as in the color shiftingamount storage units 403C, 403M, 403Y, and 403K in the first and secondembodiments, as described above. Therefore, processing based only onthis data is the same as the first and second embodiments, and adescription thereof will be omitted.

A characteristic feature of the third embodiment lies in that a colorshifting correction amount is calculated in consideration of informationstored in the print profile storage unit 1420.

FIG. 17 shows the relationship between an exposure profile stored in theexposure profile storage unit 1403C and a print profile stored in theprint profile storage unit 1420.

The slope amounts based on the scanning exposure directions and thenumber of scanning beams (FIG. 17 shows that the number of beamsgenerated by the respective image forming units is 4) will be examinedusing FIGS. 18A to 18C.

FIG. 18A shows an example wherein a 1-dot line is scanned per scan, andthe scanning directions of M (magenta) and C (cyan) components areopposite to each other. FIG. 18B shows an example of 2-dot lines perscan (two pairs of laser elements and polygonal mirrors). FIG. 18C showsan example of 4-dot lines per scan.

The example of FIG. 18A will be explained below. The exposure startpositions of images are 4m for magenta, and 4c for cyan. However, sincethe scanning directions of these color components are opposite to eachother, the positions of dots upon completion of scanning of a main scanimage region are 4m′ and 4c′. Let Lmax be the moving distance (exposurerange) of a beam per scan, and mdot be the distance between dots. Then,the slope based on the above positional relationship is given by:mdot/Lmax

The slopes based on the dot positional relationships in FIGS. 18B and18C are:2 beams: 2*mdot/Lmax4 beams: 4*mdot/LmaxLet n be the number of beams used per scan. Then, the slope is given by:n*mdot/LmaxAlso, if the shifting direction in FIG. 3 is a positive, an arithmeticoperation is made by adding counting of slopes to have a negative signin the case of Forward scanning and a positive sign in the case ofReverse scanning.

FIGS. 19A to 19C show examples when print speeds are different. Theseexamples will be described using FIGS. 19A to 19C.

FIG. 19A shows an example at a normal speed, FIG. 19B shows an exampleat a half speed, and FIG. 19C shows an example at a double speed.

As shown in FIG. 19B, in case of the double speed (the rotational speedof the photosensitive drum is half the normal speed), since image outputprocessing is made in one main scan of two main scans, an arithmeticoperation is made to halve the slope coefficient of a slope calculatedbased on the number of beams.

As shown in FIG. 19C, in case of the double speed, since thephotosensitive body moves for two scans per main scan, an arithmeticoperation is made to double the slope coefficient of a slope calculatedbased on the number of beams.

If the print speed is k times a normal speed, the slope obtained basedon the number of beams and print speed is given by:k*n*mdot/Lmax

Therefore, a deviation y in the sub-scan direction from the referenceY-coordinate in all the regions as well as the exposure profile andprint profile is given, in the case of the Forward scanning direction,by:

$\begin{matrix}{y = {{{- x}*k*n*{{mdot}/L}\;\max} + {x*( {m\;{1/L}\; 1} )\mspace{160mu}( {0 \leq x < L} )}}} \\{= {{{- x}*k*n*{{mdot}/L}\;\max} + {m\;{1/{Ldot}}} +}} \\{( {x - {L/{Ldot}}} )*( {m\;{2/L}} )\mspace{315mu}( {L \leq x < {2\; L}} )} \\{= {{{- x}*k*n*{{mdot}/L}\;\max} +}} \\{{{( {{m\; 1} + {m\; 2}} )/L}\;{dot}} + {( {x - {2\;{L/{Ldot}}}} )*( {m\;{3/L}} )\mspace{85mu}( {{2\; L} \leq x \leq {3\; L}} )}}\end{matrix}$Note that calculations are made to have L2=2*L1 and L3=3*L1 in FIG. 3.

In case of the Reverse scanning direction,

$\begin{matrix}{y = {{x*k*n*{{mdot}/L}\;\max} + {x*( {m\;{1/L}\; 1} )\mspace{160mu}( {0 \leq x < L} )}}} \\{= {{x*k*n*{{mdot}/L}\;\max} + {m\;{1/{Ldot}}} +}} \\{( {x - {L/{Ldot}}} )*( {m\;{2/L}} )\mspace{315mu}( {L \leq x < {2\; L}} )} \\{= {{x*k*n*{{mdot}/L}\;\max} +}} \\{{( {{m\; 1} + {m\; 2}} )/{Ldot}} + {( {x - {2\;{L/{Ldot}}}} )*( {m\;{3/L}} )\mspace{85mu}( {{2\; L} \leq x \leq {3\; L}} )}}\end{matrix}$

In the print processing, the exposure start position differs dependingon the paper sizes. That is, the offset position of the X-address mustbe changed. For this reason, y used in the coordinate conversionprocessing in the sub-scan direction of an image starts from Yobj at theoffset position. A correction amount in the vertical direction at theoffset position can be calculated using a formula used to calculate y.

Therefore, when the arrangement shown in FIG. 3 is adopted, the positionof each region may be set in the register 82 shown in FIG. 11 dependingon as to whether or the exposure direction of each image forming unit isForward or Reverse, and a composite slope based on the exposure andprint profiles may be set in the register 84.

As described above, according to the third embodiment which is made tosolve the aforementioned problems, color shifting correction is donefirst by calculating the read coordinate position of image data to beprinted on the basis of shifting amount information indicating theshifting amount with respect to the scanning direction on the imagecarrier of each image forming unit. After that, halftone processing isthen executed to print an image, thus suppressing generation of moirédue to color shifting correction. Furthermore, as for an edge of acharacter/line image, generation of jaggedness can be suppressed, and ahigh-quality image can be formed.

<Fourth Embodiment>

In the first to third embodiments, the example in which the address usedto load image data from each of the bitmap memories 406C, 406M, 406Y,and 406K is generated by the arrangement shown in FIG. 11 has beenexplained.

When the arrangement shown in FIG. 11 is adopted, every time theX-address is updated, Δy including a decimal point must be cumulativelycounted. Once scanning exposure for one page starts, the offset amount(integer part, decimal part) to the Y-axis remains the same as long asif the X-coordinate is the same for respective scans. Hence, Y-axisoffset addresses and weighting coefficients may be calculated in advanceby arithmetic operations, and are stored in a table. Upon actual scans,the weighting coefficients for coordinate conversion and tone correctionmay be read out with reference to this table upon processing.

FIG. 20 shows the arrangement of the coordinate counter 801 uponimplementing such processing, and FIG. 21 shows the flow of processingassociated with that arrangement.

As described above, this arithmetic processing need only be determinedonce depending on the engine state (including a print mode). A CPU (notshown) in this image processing apparatus executes the arithmeticprocessing, and stores that result in a correction arithmetic table 623.This write process is made upon starting up the image processingapparatus or upon changing the print speed. A selector 622 supplies atable lookup address 65 as a table address 64 to the correctionarithmetic table 623 when the CPU (not shown) requires access to thecorrection arithmetic table 623. When the CPU does not make any access,a coordinate address from an adder 621 is used as the table address 64.At this time, a register 620 which stores an offset value is set with anoffset (01, 02, 03, or the like in FIG. 17) depending on the printmedium size and orientation.

When the print processing starts, since the size and orientation ofsheets to be printed are determined, the CPU (not shown) sets the offsetof the X-address by writing it as offset data 610 in the offset valueregister 620.

In the above arrangement, the CPU writes the integer parts of the sumsof slopes Δy and weighting coefficients α and β in turn from theX-address offset in the correction arithmetic table 623. In the tablebelow, assume that the slope Δy=+0.2.

Write Y-address Weighting Weighting address offset coefficient αcoefficient β 0 0 0.0 1.0 1 0 0.2 0.8 2 0 0.4 0.6 3 0 0.6 0.4 4 0 0.80.2 5 1 0.0 1.0 6 1 0.2 0.8 7 1 0.4 0.6 8 1 0.6 0.4 . . . . . . . . . .. .

The coordinate counter 801 supplies a corresponding Y-address offsetvalue to the coordinate converter 802 in accordance with the X-address.At the same time, the coordinate counter 801 outputs the values α and βto the tone corrector 804. As a result, since the coordinate converter802 can obviate the need for the addition processing including thedecimal point, and the tone corrector 804 need not execute processingfor calculating α and β, the load can be reduced.

The processing in this embodiment upon implementing the aforementionedprocessing can be processed according to the flowchart of FIG. 21. Theprocessing for only the C component will be described below, but thesame applies to other components. Note that a description will be givenwith reference to the third embodiment.

In step S1701, an exposure profiles are loaded from the exposure profilestorage unit 1403C (the same applies to the units 1403M, 1403Y, and1403K). In step S1702, a print profile is loaded from the print profilestorage unit 1420.

After that, the flow advances to step S1703, and correction data basedon these profiles (X-address offset values, Y-address offset values, andweighting coefficients α and β) are calculated in consideration of theprint mode (the size and conveying direction of print sheets, printspeed, and the like). In step S1704, these calculated data are stored atcorresponding address positions of the correction arithmetic table 623.

It is checked in step S1705 if the print mode is changed. If it isdetermined that the print mode is changed, the processes in step S1703and S1704 are executed again. That is, the contents of the correctionarithmetic table 623 are updated.

If it is detected in step S1706 that print processing starts, the flowadvances to step S1707 to load the offset value from the correctionarithmetic table 623. In step S1708, coordinate data are determined. Instep S1709, data at the corresponding coordinate position is read outfrom the bitmap memory 406C. In step S1710, correction processing(interpolation processing, exception processing) is executed. Then, theprocesses in step S1708 and subsequent steps are repeated until it isdetermined in step S1711 that the print processing is complete.

As the correction arithmetic processing in step S1703 and the writeprocessing in step S1704 in the above processes, processing shown inFIG. 22 can be executed. A description will be given with reference toFIG. 22.

In steps S1801 and S1802, an exposure profile and print profile areloaded. In step S1803, a variable x indicating the X-address is reset to“0”.

After that, in step S1804, the Y-address offset value, and weightingcoefficients α and β for the variable x are calculated. In step S1805,the calculated data are written in the correction arithmetic table 623.After that, it is checked in step S1806 if the Y-address offset valueexceeds a variable ymax that holds a maximum offset (which is reset tozero in an initial state). If it is determined that the offset valueexceeds ymax, ymax is updated by the Y-address offset value at that time(step S1807).

It is checked in step S1808 if the offset arithmetic operations for oneline are complete by comparing the variable x at that time with an endcoordinate xend of one line. If NO in step S1808, the variable x isincremented by “1” in step S1809, and the processes in step S1804 andsubsequent steps are repeated.

If it is determined that the offset arithmetic operations for one lineare complete, the flow advances to step S1810 to check if the finalY-axis offset value ymax exceeds “1”. If NO in step S1810, since nocorrection is required, the flow advances to step S1811 to write allzeros in the correction arithmetic table 623.

<Fifth Embodiment>

When the color conversion unit 405 instructs that print information ofinterest indicates print processing using a single color, i.e., only oneimage forming unit, no color shifting occurs. Therefore, in suchsituation, respective profiles may be ignored, and “0”s may beunconditionally written in the correction arithmetic table.

In order to check whether or not to execute color shifting amountcorrection, a value used to evaluate the maximum value of ymax isadditionally set in each color shifting amount arithmetic unit. If ymaxis larger than this evaluation value, color shifting correction isexecuted even for single-color print processing.

FIG. 23 is a block diagram for implementing such processing in place ofFIG. 20.

In FIG. 23, signals 91 to 98 and components 920 to 923 are the same asthe signals 61 to 68 and components 620 to 623 in FIG. 20. A differencefrom FIG. 20 is that a maximum value detector 928 for detecting ymax, aregister 926 for storing a boundary value used to determine whether ornot to execute color shifting correction, a determination unit 927, anda selector 925 are arranged.

That is, when a single color mode is selected, and data from the maximumvalue detector 928 is equal to or smaller than data in the register 926,the determination unit 927 controls the selector 925 to unconditionallyoutput “0” so as to inhibit color shifting correction. Under otherconditions, the determination unit 927 controls the selector 925 toselect data from the correction arithmetic table 923.

<Sixth Embodiment>

In the description of the third embodiment, exposure profile informationis written in each of the exposure profile storage units 1403C, 1403M,1403Y, and 1403K in the factory manufacturing process. However, suchinformation may be varied from that upon factory shipment due to agingsince the apparatus includes many mechanical operation components andthe like.

Hence, the sixth embodiment will exemplify a case wherein the controller402 side writes and updates each exposure profile storage unit 1403. Torewrite the contents, the apparatus comprises a circuit for writinginformation in the exposure profile storage unit 1403. However, sincesuch circuit is known to those who are skilled in the art, a descriptionthereof will be omitted. In order to update the exposure profile,detection of a color shifting amount of the exposure unit will beexplained.

In the sixth embodiment, as shown in FIG. 24, a 1-line dot pattern isexposed on a pre-exposure region (which is not used in normal printprocessing and has a length (the number of dots) Lpat) of thephotosensitive drum, and is transferred onto a print sheet. After that,the coordinate positions of the right and left ends are detected. Atthis time, if the photosensitive drum is normal, the detection timingsof patterns 2009 and 2008 at the right and left ends of the 1-dot linediffer just by the length according to the print profile. That is, incase of the normal photosensitive drum, these patterns are detected attimings having a difference k*m/Lpat.

Therefore, a result obtained by subtracting “k*m/Lpat” is the shiftingamount between the right and left ends of the exposure profile at thattime. In this embodiment, since shifting amounts at the positions offour points including the two ends are calculated, as shown in FIG. 3,two central points are re-set (overwritten) since they are differentfrom shifting amounts upon factory shipment at the same ratio as thatfor the two end points.

As a result, since the exposure profile is updated, generation of colorshifting can be suppressed in correspondence with aging. Note that theexposure profile is updated when an instruction is input from a controlpanel (not shown).

>Seventh Embodiment>

FIG. 25 is a block diagram of the seventh embodiment in place of FIG. 20described above.

In this arrangement, since print profile data is handled as fixedcoefficients, processing is done using that information which changesdepending on print processing. With this arrangement, when the exposureprofiles are set once upon starting up the apparatus, and the printprofile value is changed depending on the internal state, the objectiveprocessing can be achieved. Note that reference numerals 1101 to 1108 inFIG. 25 denote signals which are the same as the signals 61 to 68 inFIG. 20, and reference numerals 1120 to 1123 denote building componentswhich are the same as the components 620 to 623 in FIG. 20. A differenceis that an adder 1125, multiplier 1127, and register 1126 for holdingprint profile coefficients are added to FIG. 20.

FIG. 26 shows the processing flow in this example. In this case,processing for only the C component will be explained, but the sameapplies to other components.

In step S2201, an exposure profile is loaded from the exposure profilestorage unit 1403C. A color shifting correction amount is calculatedbased on the exposure profile in step S2202, and the arithmetic resultis written in the exposure profile correction arithmetic table 1123 fortemporary storage in step S2203.

After that, the flow advances to step S2204 to acquire a print profilefrom the print profile storage unit 1420 to generate a print profile inconsideration of the print mode (the size and conveying direction ofprint sheets, print speed, and the like). In step S2205, the generatedprint profile is stored in the register 1126 as a temporary printprofile coefficient.

It is checked in step S2206 if the print mode is changed. If it isdetermined that the print mode is changed, the processes in steps S2204and S2205 are repeated. That is, the contents to be updated are those ofonly the register 1126.

If it is detected in step S2207 that print processing starts, the flowadvances to step S2208 to load the offset value from the table 1123. Instep S2209, coordinate data are determined. In step S2210, data at thecorresponding coordinate position is read out from the bitmap memory406C. In step S2211, correction processing (interpolation processing,exception processing) is executed. Then, the processes in step S2209 andsubsequent steps are repeated until it is determined in step S2212 thatthe print processing is complete.

The preferred embodiments according to the present invention have beenexplained. The arrangement shown in FIG. 16 may be implemented bysoftware (firmware). In this case, processing that allows image data toflow according to FIG. 16 can be implemented, and such implementation iseasy for those who are skilled in the art from the description of thisembodiment.

As many apparently widely different embodiments of the present inventioncan be made without departing from the spirit and scope thereof, it isto be understood that the invention is not limited to the specificembodiments thereof except as defined in the claims.

CLAIM OF PRIORITY

This application claims priorities from Japanese Patent Applications No.2004-350302 filed on Dec. 2, 2004, No. 2004-350303 filed on Dec. 2, 2004and No. 2004-350304 filed on Dec. 2, 2004, which are hereby incorporatedby reference herein.

What is claimed is:
 1. An image processing apparatus comprising: amemory constructed to store information indicating a shifting amount,from a base line, in a sub scanning direction of a curved scanning linedrawn on an image carrier of a print engine; a controlling part havingone or more processors which execute instructions, the controlling partcomprising: a correction unit constructed to perform, on a pixelincluded in image data, correction process of a tone value of the pixelfor smoothing a jaggy which results from a shift process of shifting thepixel on a basis of the shifting amount indicated by the storedinformation; a determination unit constructed to determine an imageattribute of a pixel included in the image data; and a control unitconstructed to control the correction unit to perform the correctionprocess on a pixel included in the image data whose image attribute isdetermined to be a first image attribute, and not to perform thecorrection process on a pixel included in the image data whose imageattribute is determined to be a second image attribute, wherein thecorrection unit is constructed to perform the correction process byderiving a weighted mean of tone values of two adjacent pixels in thesub-scanning direction included in the image data using a weightcoefficient based on the shifting amount indicated by the storedinformation.
 2. The image processing apparatus according to claim 1,wherein the determination unit is constructed to determine the imageattribute of the pixel included in the image data by pattern-matching aplurality of pixels included in the image data with a predeterminedpattern; and the control unit is constructed to control the correctionunit to perform the correction process on a pixel included in the imagedata whose image attribute is determined to be a first image attribute,and not to perform the correction process on a pixel included in theimage data whose image attribute is determined to be a second imageattribute.
 3. The image processing apparatus according to claim 1,wherein the determining unit performs the determination bypattern-matching a plurality of pixels included in the image data with apredetermined pattern.
 4. The image processing apparatus according toclaim 1, wherein the first image attribute corresponds to a line.
 5. Theimage processing apparatus according to claim 1, wherein the secondimage attribute corresponds to a photo.
 6. The image processingapparatus according to claim 1, further comprising the print engineconstructed to print an image based on the image data including thepixel on which the correction process has been performed by thecorrection unit.
 7. An image processing apparatus comprising: a memoryconstructed to store image data; a controlling part having one or moreprocessors which execute instructions, the controlling part comprising:a pattern matching unit constructed to perform pattern-matching fordetermining whether or not a plurality of pixels included in the imagedata have a predetermined pattern; and a correction unit constructed toperform, on one of the pixels, according to a result of thepattern-matching, a correction process of a tone value of the one of thepixels for smoothing a jaggy, the jaggy resulting from a shift processof shifting the one of the pixels in a sub-scanning direction; whereinthe image processing apparatus further comprises: a print engineconstructed to print an image based on the image data including the oneof the pixels on which the correction process has been performed,wherein the shift process is performed based on a shifting amount, froma base line, in the sub-scanning direction of a curved scanning linedrawn on an image carrier of the print engine, and wherein thecorrection unit is constructed to perform the correction processing byderiving a weighted mean of tone values of two adjacent pixels in thesub-scanning direction included in the image data using a weightcoefficient determined based on the shifting amount.
 8. The imageprocessing apparatus according to claim 7, wherein the correction unitis constructed be controlled, according to a result of thepattern-matching, to perform or not to perform the correction process.9. The image processing apparatus according to claim 8, wherein thecorrection unit is constructed to be controlled to perform thecorrection process according to a result of the pattern-matching being afirst result, and not to perform the correction process according to aresult of the pattern-matching being a second result different from thefirst result.
 10. The image processing apparatus according to claim 7,wherein the correction unit is constructed to determine, according to aresult of the pattern-matching, whether or not to perform the correctionprocess, and wherein the correction unit is constructed to perform thecorrection process if the correction unit determines to perform thecorrection process, and not to perform the correction process if thecorrection unit determines not to perform the correction process. 11.The image processing apparatus according to claim 7, wherein thepredetermined pattern corresponds to a line.
 12. An image processingapparatus comprising: a memory constructed to store image data; acontrolling part having one or more processors which executeinstructions, the controlling part comprising: an acquisition unitconstructed to acquire a pixel attribute of a pixel included in theimage data from the image data; and a correction unit constructed toperform, on the pixel, according to the pixel attribute of the pixel, acorrection process of a tone value of the pixel for smoothing a jaggy,the jaggy resulting from a shift process of shifting the pixel in asub-scanning direction; wherein the image processing apparatus furthercomprises: a print engine constructed to print an image based on theimage data including the pixel on which the correction process has beenperformed, wherein the shift process is performed based on a shiftingamount, from a base line, in the sub-scanning direction of a curvedscanning line drawn on an image carrier of the print engine, and whereinthe correction unit is constructed to perform the correction process byderiving a weighted mean of tone values of two adjacent pixels in thesub-scanning direction included in the image data using a weightcoefficient determined based on the shifting amount.
 13. The imageprocessing apparatus according to claim 12, wherein the acquisition unitis constructed to acquire the pixel attribute of the pixel by analyzingthe image data.
 14. The image processing apparatus according to claim12, wherein the acquisition unit is constructed to acquire the pixelattribute of the pixel by performing pattern-matching for determiningwhether or not a portion, which includes the pixel, included in theimage data has a predetermined pattern.
 15. The image processingapparatus according to claim 12, wherein the correction unit isconstructed to be controlled, according to the pixel attribute of thepixel, to perform or not to perform the correction process.
 16. Theimage processing apparatus according to claim 15, wherein the correctionunit is constructed to be controlled to perform the correction processaccording to the pixel attribute being a first pixel attribute, and notto perform the correction process according to the pixel attribute beinga second pixel attribute different from the first pixel attribute. 17.The image processing apparatus according to claim 16, wherein the firstpixel attribute corresponds to a line and the second pixel attributecorresponds to a photo.
 18. The image processing apparatus according toclaim 12, wherein the correction unit is constructed to perform thecorrection process according to the pixel attribute being a first pixelattribute, and not to perform the correction process according to thepixel attribute being a second pixel attribute.
 19. The image processingapparatus according to claim 18, wherein the first pixel attributecorresponds to a line.
 20. The image processing apparatus according toclaim 18, wherein the second pixel attribute corresponds to a photo. 21.The image processing apparatus according to claim 12, wherein the shiftprocess is performed based on a shifting amount, from a base line, inthe sub-scanning direction of a curved scanning line drawn on an imagecarrier of the print engine, and wherein the correction unit isconstructed to perform the correction process by deriving a weightedmean of tone values of two adjacent pixels in the sub-scanning directionincluded in the image data using a weight coefficient determined basedon the shifting amount.
 22. An image processing apparatus comprising: amemory constructed to store image data; a controlling part having one ormore processors which execute instructions, the controlling partcomprising: an acquisition unit constructed to acquire a pixel attributeof a pixel included in the image data; and a correction unit constructedto perform, on the pixel, a correction process of a tone value of thepixel for smoothing a jaggy, the jaggy resulting from a shift process ofshifting the pixel in a sub-scanning direction and the shift processbeing performed according to shift information relating to shiftamounts, from a base line, in the sub-scanning direction of a curvedscanning line drawn on an image carrier of a print engine, wherein thecorrection unit is constructed to control whether or not to perform thecorrection process on the pixel, according to the pixel attribute of thepixel and the shift information.
 23. The image processing apparatusaccording to claim 22, wherein the acquired pixel attribute relates towhether or not to perform the correction process.
 24. The imageprocessing apparatus according to claim 22, wherein the correction unitis constructed to perform the shift process.
 25. The image processingapparatus according to claim 22, further comprising the print engine.26. The image processing apparatus according to claim 22, wherein theacquisition unit is constructed to acquire the pixel attribute bypattern-matching.
 27. A method for processing image data to form animage by scanning a rotatable photoconductive member with a laser beamcomprising: storing image data; identifying a type of an image of theimage data; and controlling whether or not to perform a correctionprocess of a tone value of the image data for compensating for adeviation less than one pixel of the scanning in a rotation direction ofthe photoconductive member, according to information relating to thedeviation of the scanning and the identified type of the image.
 28. Themethod according to claim 27, wherein the image data corresponds to apart of an image of a page.
 29. The method according to claim 27,wherein a scan line by the scanning is curved by an optical system forthe laser beam.
 30. The method according to claim 27, wherein theidentifying includes: determining whether or not a pattern of the imageof the image data matches a predetermined pattern; and identifying thetype of the image of the image data according to the determinationresult of the matching.
 31. The method according to claim 27, whereinthe identified type of the image is one of a first type of an image ofimage data on which the correction process is performed and a secondtype of an image of image data on which the correction process is notperformed.
 32. The method according to claim 31, wherein the first typeof an image corresponds to a line-like image, and the second type of animage corresponds to a photo-like image.
 33. The method according toclaim 27, further comprising: performing a shift process of shifting theimage data in a pixel unit in the rotation direction according toinformation relating to a deviation of an integer pixel of the scanning,wherein the correction process corrects a jaggy which results from theperforming of the shift process.
 34. The method according to claim 27,further comprising: forming an image based on the image data on whichthe correction process is performed or not performed by the controlling.35. The method according to claim 27, wherein the correction processcorrects a tone value of a pixel in the image data based on the tonevalue of the pixel and a tone value of another pixel next to the pixelin the rotation direction.
 36. An image processing apparatus forprocessing image data to form an image by scanning a rotatablephotoconductive member with a laser beam comprising: a memory; and atleast one processor, which is coupled with the memory, configured tocontrol the image processing apparatus, the image processing apparatusbeing configured to: store image data; identify a type of an image ofthe image data; and control whether or not to perform a correctionprocess of a tone value of the image data for compensating for adeviation less than one pixel of the scanning in a rotation direction ofthe photoconductive member, according to a shift of the scanning and theidentified type of image.
 37. The image processing apparatus accordingto claim 36, wherein a scan line by the scanning is curved by an opticalsystem for the laser beam.
 38. The image processing apparatus accordingto claim 36, wherein the identification is performed by: determiningwhether or not a pattern of the image of the image data matches apredetermined pattern; and identifying the type of the image of theimage data according to the determination result of the matching. 39.The image processing apparatus according to claim 36, wherein theidentified type of the image is one of a first type of an image of imagedata on which the correction process is performed and a second type ofan image of image data on which the correction process is not performed.40. The image processing apparatus according to claim 39, wherein thefirst type of an image corresponds to a line-like image, and the secondtype of an image corresponds to a photo-like image.
 41. The imageprocessing apparatus according to claim 36, wherein the image processingapparatus, which is controlled by the at least one processor, isconfigured to perform a shift process of shifting the image data in apixel unit in the rotation direction according to information relatingto a deviation of an integer pixel of the scanning, wherein thecorrection process corrects a jaggy which results from the performing ofthe shift process.
 42. The image processing apparatus according to claim36, further comprising: a print engine configured to print an imagebased on the image data on which the correction process is performed ornot performed by the control.
 43. The image processing apparatusaccording to claim 36, wherein the correction process corrects a tonevalue of a pixel in the image data based on the tone value of the pixeland a tone value of another pixel next to the pixel in the rotationdirection.
 44. An image processing method for processing image databased on shift information relating to a shift of an image which isformed by an electrophotographic image forming device: shifting,according to the shift information, one of two groups of pixelscontained in a main scanning line of the image data by one pixel in asub scanning direction with respect to another of the two groups, thetwo groups being adjacent to each other; identifying an image type ofeach of pixels contained in the one group; processing, according to theidentified image type, the contained pixels which have been shiftedaccording to the shift information, wherein the processing includes:correcting a tone value of the contained pixel whose image type isidentified as a first image type and outputting the corrected tone valueof the contained pixel; and outputting, without the correcting, a tonevalue of the contained pixel whose image type is identified as a secondimage type different form the first image type.
 45. The image processingmethod according to claim 44, wherein the electrophotographic imageforming device includes a photoconductive member, an optical system, anda laser scanner for scanning the photoconductive member with a laserbeam through the optical system to form the image, and wherein the shiftis due to at least a scan line of the scanning curved by the opticalsystem.
 46. The image processing method according to claim 44, whereinthe correcting is performed for shifting an image by an amount less thanone pixel in the sub scanning direction.
 47. The image processing methodaccording to claim 44, wherein the correcting is performed by blendingthe tone value of the contained pixel whose image type is identified asthe first image type and a tone value of another pixel adjacent to thecontained pixel in the sub scanning direction.
 48. The image processingmethod according to claim 47, wherein a ratio of the blending isdetermined based on a position in the one group of the contained pixelwhose image type is identified as the first image type.
 49. Anelectrophotographic image forming apparatus, comprising: a photoconductive member which is subject to a laser beam scan; a memory whichstores information relating to a curve of the laser beam scan; and atleast a processor, a circuit or the combination of the processor and thecircuit which serves as: a shifting unit which shifts, at a shiftposition determined based on the information, a pixel contained in amain scanning line of image data by one pixel in a sub scanningdirection with respect to another pixel contained in the main scanningline and adjacent to the pixel; an identifying unit which identifies animage type of the pixel which is shifted by the shifting unit; and acorrecting unit which can perform a tone value correction for the pixelwhich is shifted by the shifting unit, wherein the correcting unitdetermines, based on the identified image type, whether or not toperform the tone correction for the pixel which is shifted by theshifting unit.
 50. The electrophotographic image forming apparatusaccording to claim 49, wherein the tone value correction performed bythe correcting unit is based on the shift position at which the shiftingunit shifts the pixel.
 51. The electrophotographic image formingapparatus according to claim 49, wherein the correcting unit performsthe tone value correction for shifting an image by an amount less thanone pixel in the sub scanning direction.
 52. The electrophotographicimage forming apparatus according to claim 49, wherein the correctingunit performs the tone value correction by blending the tone value ofthe contained pixel whose image type is identified as the first imagetype and a tone value of another pixel adjacent to the contained pixelin the sub scanning direction.